I'd second the motion for Systema and for the Filipino arts, and toss in a nod for Shindo-ryu karate (such as that of Kenji Ushiro Sensei). There will doubtless be other good choices as well. The right match for you, however, is very much a function of the style of Aikido you practice. There are many different styles, and within them, many different teachers. A certain art might make good cross-training sense for some, whereas others might benefit more from a different source. Look around. Find what feels best for you. It's not a race, so take your time and find the right fit.